FIGS. 1A-1C illustrate the current state of the art in developing at least one type of multilayer electronic structure. A core consists of at least one layer of copper, a layer of bonding film (e.g., FR4, etc.), and a second layer of copper. Selected locations of the second layer of copper are removed (e.g., etched), leaving intact copper signal traces that provide for the internal circuitry of the electronic structure. Bonding film is laminated between a first core and a second core to provide a continuous multilayer electronic structure. When signal trace density changes (i.e., in a first location on the core there are numerous signal traces, and in a second location there are very few, if any, signal traces,) the distance from the signal traces to the reference ground layer varies across the PCB. This variation of distance results in variations in mechanical thickness, impedance, and electrical performance of the multilayer electronic structure.
For example see FIG. 1A. FIG. 1A depicts a prior art multilayer electronic structure having a single isolated signal trace. For instance if the signal trace is 4 mils wide, 0.7 mils thick, and the bonding film is 4 mils thick, after lamination the distance from the top of the signal trace to the adjacent reference ground layer approaches 3.3 mils.
Alternatively see FIG. 1B. FIG. 1B depicts a prior art multilayer electronic structure having a signal trace nestled between two wide traces (e.g., power signal trace, ground signal trace, etc.). For instance if the signal trace is 4 mils wide, 0.7 mils thick, and the bonding film is 4 mils thick, after lamination the distance from the top of the signal trace to the adjacent reference ground layer approaches 4.0 mils.
Alternatively see FIG. 1C. FIG. 1C depicts a prior art multilayer electronic structure having a single trace nestled between two other signal traces. For instance if each signal trace is 4 mils wide, 0.7 mils thick, and the bonding film is 4 mils thick, after lamination the distance from the top of the signal traces to the adjacent reference ground layer approaches 3.65 mils.
In the examples depicted in FIGS. 1A, 1B, and 1C, the distance from the top of the signal traces to the adjacent reference ground layer by itself leads to impedance differences of 48-51-53 Ohms respectively.
In the current state of the art, impedance and mechanical (i.e., thickness) tolerance requirements are tight and may become tighter. Currently impedance tolerances of +/−10% are typical (e.g., 50 Ohms +/−5 Ohms). In the future, impedance tolerances of +/−7.5% or 5.0% may become more common. In the examples depicted in FIGS. 1A, 1B, and 1C, 50% of the +/−10.0% tolerance is taken up by the effect of the distance variations from the top of the signal trace to the adjacent reference ground layer.